As body frame structures, those are now in practical use which have a subframe which supports drive train parts including a power source, steering system parts such as a steering gear box and suspension system parts such as suspensions, and camber and caster angles adjusting mechanisms.
In those body frame structures which are now in practical use, it has been practically sufficient that a front subframe is formed substantially in parallel crosses or into a rectangle in such a manner that drive train parts, steering system parts or suspension system parts can be installed thereon.
As these body frame structures, those are known which adopt a front subframe which is made up of a die-cast product using an aluminum alloy as a material and a front subframe which is made up of an aluminum allow extruded product (for example, refer to JP-A-2002-137617 (page 4, FIG. 1), JP-A-2000-177621 (Page 5, FIG. 2)).
FIG. 20 is a drawing which explains a basic configuration of a conventional body frame structure having a subframe made of an aluminum die-cast product, and a body frame structure 300 is made up of a subframe 301 that is attached to the body side, an upper arm 302 which extends outwards from an upper portion of the subframe 301 in such a manner as to freely swing, lower arms 303 which extend outwards from lower portions of the subframe 301 in such a manner as to freely swing, and an axle support member (a knuckle) 307 which supports an axle (not shown) by being movably attached to distal ends of the upper arm 302 and the lower arms 303, 303 via an upper ball joint 304 and lower ball joints 305, 305, respectively, and is formed integrally through die casting an aluminum alloy, wherein the subframe 301 is formed in parallel crosses.
FIG. 21 is a drawing which explains a basic configuration of a conventional body frame having a subframe which is made of an aluminum alloy extruded product, and a body frame structure 310 is a subframe structure (a front subframe) 311 which supports drive train parts including a power source and suspension system parts such as suspensions.
The subframe 311 is made up of a front frame portion 312, left and right front corner portions 313, 313 which are connected to left and right end portions of the front frame portion 312, a left frame portion 314 which extends rearwards from the left front corner portion 313, a right frame portion 314 which extends rearwards from the right front corner portion 313, left and right rear corner portions 315, 315 which are connected to distal ends of the left and right frame portions 314, 314 and a rear frame portion 316 which is connected to the left and right rear corner portions 315, 315, respectively.
In addition, the subframe 311 utilizes aluminum alloy extruded products at the front frame portion 312 and the rear frame portion 316, the left and right frame portions 314, 314, the left and right front corner portions 313, 313 and the left and right rear corner portions and is formed substantially into a rectangular shape.
In the body frame structure 300 shown in FIG. 20, however, since the subframe 301 is such as to be formed integrally by die casting the aluminum alloy, the rigidity of the whole frame is high, and for example, in order to increase the load transmission performance which disperses and absorbs an excessive load such as an impact which is applied to the subframe 301, when such a thing occurs, and the shape maintaining performance which maintains an initial shape of the subframe 301, the shape of the subframe 301 needs to be complex, and this leads to a problem that such a complex shape makes it difficult to increase the productivity under mass production.
In addition, in the body frame structure 310 shown in FIG. 21, since the subframe 311 is made up of the aluminum alloy extruded product, the rigidity of the whole frame is low, and in order to increase the rigidity at, for example, a fixing portion of such as a steering gear box (not shown) where a large steering reaction is generated, a fixing portion such as a fixing portion of a suspension where there is a large input and a connecting portion to the body where road surface vibrations are inputted, the thickness of those portions needs to be increased, leading to a drawback that the increase in thickness calls for an increase in vehicle weight.
In addition, in the body frame structure 310, since the subframe (the front subframe) is divided into the front frame portion 312 and the rear frame portion 316, the left and right frame portions 314, 314, the left and right front corner portions 313, 313 and the left and right rear corner portions 315, 315, there occur assembling errors when these members are assembled, leading to a problem that the body frame structure 310 is not suitable for locations where dimensional accuracy is required.
Namely, a body frame structure is desired which can increase the load transmission performance and the shape maintaining performance while realizing the suppression of increase in vehicle weight.
In addition, a body frame structure is desired which can secure the rigidity of the frame while realizing the suppression of increase in vehicle weight.
Furthermore, a body frame structure is desired which can not only suppress the increase in vehicle weight while increasing the rigidity of the frame but also increase the accuracy at locations where dimensional accuracy is required.